Longitudinally flexible stent

ABSTRACT

An intravascular stent especially suited for implanting in curved arterial portions. The stent retains longitudinal flexibility after expansion. The stent is formed of intertwined meander patterns forming triangular cells. The triangular cells are adapted to provide radial support, and also to provide longitudinal flexibility after expansion. The triangular cells provide increased coverage of a vessel wall. The stent can have different portions adapted to optimize radial support or to optimize longitudinal flexibility. The stent can be adapted to prevent flaring of portions of the stent during insertion.

FIELD OF THE INVENTION

[0001] The present invention relates generally to stents, which areendoprostheses implanted into vessels within the body, such as bloodvessels, to support and hold open the vessels, or to secure and supportother endoprostheses in the vessels. In particular, the presentinvention relates to a stent which is longitudinally flexible before andafter expansion.

BACKGROUND OF THE INVENTION

[0002] Various stents are known in the art. Typically stents aregenerally tubular in shape, and are expandable from a relatively small,unexpanded diameter to a larger, expanded diameter. For implantation,the stent is typically mounted on the end of a catheter, with the stentbeing held on the catheter at its relatively small, unexpanded diameter.By the catheter, the unexpanded stent is directed through the lumen tothe intended implantation site. Once the stent is at the intendedimplantation site, it is expanded, typically either by an internalforce, for example by inflating a balloon on the inside of the stent, orby allowing the stent to self-expand, for example by removing a sleevefrom around a self-expanding stent, allowing the stent to expandoutwardly. In either case, the expanded stent resists the tendency ofthe vessel to narrow, thereby maintaining the vessel's patency.

[0003] U.S. Pat. No. 5,733,303 to Israel et al. (“'303”), which isexpressly incorporated by reference, shows a unique stent formed of atube having a patterned shape which has first and second meanderpatterns having axes extending in first and second directions. Thesecond meander patterns are intertwined with the first meander patternsto form flexible cells. Stents such as this one are very flexible intheir unexpanded state such that they can be tracked easily downtortuous lumens. Upon expansion, these stents provide excellent radialsupport, stability, and coverage of the vessel wall. These stents arealso conformable, in that they adapt to the shape of the vessel wallduring implantation.

[0004] One feature of stents with a cellular mesh design such as thisone, however, is that they have limited longitudinal flexibility afterexpansion, which may be a disadvantage in particular applications. Thislimited longitudinal flexibility may cause stress points at the end ofthe stent and along the length of the stent. Conventional mesh stentslike that shown in U.S. Pat. No. 4,733,665 may simply lack longitudinalflexibility, which is illustrated by FIG. 1, a schematic diagram of aconventional stent 202 in a curved vessel 204.

[0005] To implant a stent, it maybe delivered to a desired site by aballoon catheter when the stent is in an unexpanded state. The ballooncatheter is then inflated to expand the stent, affixing the stent intoplace. Due to the high inflation pressures of the balloon—up to 20atm—the balloon causes the curved vessel 204 and even a longitudinallyflexible stent to straighten when it is inflated. If the stent, becauseof the configuration of its mesh is or becomes relatively rigid afterexpansion, then the stent remains or tends to remain in the same orsubstantially the same shape after deflation of the balloon. However,the artery attempts to return to its natural curve (indicated by dashedlines) in FIG. 1 with reference to a conventional mesh stent. Themismatch between the natural curve of the artery and the straightenedsection of the artery with a stent may cause points of stressconcentration 206 at the ends of the stent and stress along the entirestent length. The coronary vasculature can impose additional stress onstents because the coronary vasculature moves relatively significantamounts with each heartbeat. For illustration purposes, the differencebetween the curve of the vessel and the straightened stent has beenexaggerated in FIG. 1.

[0006] U.S. Pat. No. 5,807,404 to Richter, which is expresslyincorporated by reference, shows another stent which is especiallysuited for implantation into curved arterial portions or ostial regions.This stent can include sections adjacent the end of the stent withgreater bending flexibility than the remaining axial length of thestent. While this modification at the end of the stent alleviates thestress at the end points, it does not eliminate the stress along theentire length of the stent.

[0007] Various stents are known that retain longitudinal flexibilityafter expansion. For example, U.S. Pat. Nos. 4,886,062 and 5,133,732 toWiktor (“the Wiktor '062 and '732 patents”) show various stents formedof wire wherein the wire is initially formed into a band of zig-zagsforming a serpentine pattern, and then the zig-zag band is coiled into ahelical stent. The stents are expanded by an internal force, for exampleby inflating a balloon.

[0008] The coiled zig-zag stents that are illustrated in FIGS. 1 through6 of the Wiktor '062 and '732 patents are longitudinally flexible bothin the expanded and unexpanded condition such that they can be trackedeasily down tortuous lumens and such that they conform relativelyclosely to the compliance of the vessel after deployment. While thesestents are flexible, they also have relatively unstable support afterexpansion. Furthermore, these stents leave large portions of the vesselwall uncovered, allowing tissue and plaque prolapse into the lumen ofthe vessel.

[0009] Thus, it is desired to have a stent which exhibits longitudinalflexibility before expansion such that it can easily be tracked downtortuous lumens and longitudinal flexibility after expansion such thatit can comply with the vessel's natural flexibility and curvature whilestill providing continuous, stable coverage of a vessel wall that willminimize tissue sag into the lumen.

OBJECTS AND SUMMARY OF THE INVENTION

[0010] Accordingly, an object of the invention is to provide a stentthat is longitudinally flexible before expansion so that it can easilybe tracked down tortuous vessels and remains longitudinally flexibleafter expansion such that it will substantially eliminate any stresspoints by complying with the vessel's flexibility and assuming thenatural curve of the vessel.

[0011] Another object of the present invention is to provide a stentthat is longitudinally flexible after delivery such that it flexesduring the cycles of the heartbeat to reduce cyclic stress at the endsof the stent and along the stent.

[0012] Another object of the present invention is to provide a stentwith a closed cell pattern such that it provides good coverage andsupport to a vessel wall after expansion.

[0013] Other advantages of the present invention will be apparent tothose skilled in the art.

[0014] In accordance with these objects, the stent of the presentinvention is formed to be a tube having a patterned shape which hasfirst and second meander patterns having axes extending in first andsecond direction wherein the second meander patterns are intertwinedwith the first meander patterns.

[0015] In accordance with one embodiment of the invention, theintertwined meander patterns form cells which have three points at whichthe first and second meander patterns meet each other, and which in thissense could be called triangular cells. These three cornered ortriangular cells are flexible about the longitudinal axis of the stentafter expansion. These triangular cells provide comparable scaffoldingand radial strength to that of cells formed by intertwined meanderpatterns which have four points at which the first and second patternsmeet each other, and which in this sense could be called square cells.

[0016] In another embodiment of the invention, bands of cells areprovided along the length of a stent. The bands of cells alternatebetween cells adapted predominantly to enhance radial support with cellsthat are adapted predominantly to enhance longitudinal flexibility afterexpansion.

[0017] In another embodiment of the invention, the first meanderpatterns are adapted to prevent any “flaring out” of loops of the firstmeander patterns during delivery of the stent.

[0018] A stent according to the invention retains the longitudinalflexibility associated with the '303 cellular stent in its unexpandedstate, and has increased longitudinal flexibility in the expanded state.The stent does so without sacrificing scaffolding—i.e. coverage of thevessel wall—or radial support.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows a schematic diagram of a conventional rigid stentdeployed in a curved lumen;

[0020]FIG. 2 shows a schematic diagram of a stent of the presentinvention deployed in a curved lumen;

[0021]FIG. 3 shows a pattern for a stent made in accordance with thepresent invention;

[0022]FIG. 4 shows an enlarged view of one cell of the pattern of FIG.3;

[0023]FIG. 5 shows a pattern for a stent made in accordance with thepresent invention;

[0024]FIG. 6 shows an enlarged view of one cell of the pattern of FIG.5;

[0025]FIG. 7 shows a pattern for a stent made in accordance with thepresent invention;

[0026]FIG. 8 shows an enlarged view of one cell used in the pattern ofFIG. 7;

[0027]FIG. 9 shows an enlarged view of another cell used in FIG. 7;

[0028]FIG. 10 shows a schematic comparison of a four cornered or “squarecell” and a three cornered or “triangular” cell of the presentinvention.

[0029]FIG. 11 shows a pattern for a stent constructed according to theprinciples of the invention which has variable geometry along itslength.

[0030]FIG. 12 shows another pattern for a stent constructed according tothe principles of the invention.

[0031]FIG. 13 shows another pattern for a stent constructed according tothe principles of the invention.

[0032]FIG. 14 shows the expansion of a portion of a horizontal meanderpattern built according to the principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0033]FIG. 2 shows a schematic diagram of a longitudinally flexiblestent 208 of the present invention. The stent 208 may be delivered to acurved vessel 210 by a balloon catheter, and implanted in the artery byinflating the balloon. As described before, the balloon causes theartery to straighten upon inflation of the balloon. However, upondeflation of the balloon, the stent 208 assumes the natural curve of thevessel 210 because it is and remains longitudinally flexible afterexpansion. This reduces any potential stress points at the ends of thestent and along the length of the stent. Furthermore, because the stentis longitudinally flexible after expansion, the stent will flexlongitudinally with the vessel during the cycles caused by a heartbeat.This also reduces any cyclic stress at the ends of the stent and alongthe length of the stent.

[0034]FIG. 3 shows a pattern of a stent according to the presentinvention. This pattern may be constructed of known materials, and forexample stainless steel, but it is particularly suitable to beconstructed from NiTi. The pattern can be formed by etching a flat sheetof NiTi into the pattern shown. The flat sheet is formed into a stent byrolling the etched sheet into a tubular shape, and welding the edges ofthe sheet together to form a tubular stent. The details of this methodof forming the stent, which has certain advantages, are disclosed inU.S. Pat. Nos. 5,836,964 and 5,997,973, which are hereby expresslyincorporated by reference. Other methods known to those of skill in theart such as laser cutting a tube or etching a tube may also be used toconstruct a stent which uses the present invention. After formation intoa tubular shape, a NiTi stent is heat treated, as known by those skilledin the art, to take advantage of the shape memory characteristics ofNiTi and its superelasticity.

[0035] The pattern 300 is formed from a plurality of each of twoorthogonal meander patterns which patterns are intertwined with eachother. The term “meander pattern” is taken herein to describe a periodicpattern about a center line and “orthogonal meander patterns” arepatterns whose center lines are orthogonal to each other.

[0036] A meander pattern 301 is a vertical sinusoid having a verticalcenter line 302. A meander pattern 301 has two loops 304 and 306 perperiod wherein loops 304 open to the right while loops 306 open to theleft. Loops 304 and 306 share common members 308 and 310, where member308 joins one loop 304 to its following loop 306 and member 308 joinsone loop 306 to its following loop 304.

[0037] A meander pattern 312 (two of which have been shaded forreference) is a horizontal pattern having a horizontal center line 314.A horizontal meander pattern 312 also has loops labeled 316, 318, 320,322, and between the loops of a period is a section labeled 324.

[0038] Vertical meander pattern 301 is provided in odd and even (o ande) versions which are 180° out of phase with each other. Thus, each leftopening loop 306 of meander pattern 301 o faces a right opening loop 304of meander pattern 301 e and a right opening loop 304 of meander pattern301 o faces a left opening loop 306 of meander pattern 301 e.

[0039] The horizontal meander pattern 312 is also provided in odd andeven forms. The straight sections 324 of the horizontal meander pattern312 e intersect with every third common member 310 of the even verticalmeander pattern 301 e. The straight sections 324 of the horizontalmeander pattern 312 o also intersect with every third common member 310of the odd vertical meander pattern 301.

[0040] Upon expansion of the stent, the loops of the vertical meanderpatterns 301 open up in the vertical direction. This causes them toshorten in the horizontal direction. The loops in the horizontal meanderpattern 312 open up both in the vertical direction and the horizontaldirection, compensating for the shortening of the loops of the verticalmeander patterns.

[0041] It should be noted that the horizontal meander pattern 312 in thepresent invention avoids foreshortening in a self-expanding stent in aparticularly effective manner. A self-expanding stent formed of ashape-memory alloy must be compressed from an expanded position to acompressed position for delivery. As shown in FIG. 14, because of theconfiguration of the horizontal meander pattern 312, when the stent iscompressed from an expanded position 602 to a compressed position 604,the length 606 of the horizontal meander pattern naturally shrinks.Consequently, when the stent expands, the horizontal meander patternelongates and compensates for the shortening of the vertical meanderpattern as the vertical meander pattern expands. In contrast, ahorizontal meander pattern with such shapes as N-shapes will notnaturally shrink longitudinally when compressed from an expandedposition 608 to a compressed position 610, as illustrated in FIG. 14.

[0042] A stent formed from the pattern of FIG. 3 and made of NiTi isparticularly well suited for use in the carotid artery or other lumenssubject to an outside pressure. One reason is that because the stent isformed of NiTi, it is reboundable, which is a desirable property forstents placed in the carotid artery. The other reason is that the stentof FIG. 3 offers excellent scaffolding, which is particularly importantin the carotid artery. Scaffolding is especially important in thecarotid artery because dislodged particles in the artery may embolizeand cause a stroke.

[0043]FIG. 4 is an expanded view of one flexible cell 500 of the patternof FIG. 3. Each flexible cell 500 includes: a first member 501 having afirst end 502 and a second end 503; a second member 504 having a firstend 505 and a second end 506; a third member 507 having a first end 508and a second end 509; and a fourth member 510 having a first end 511 anda second end 512. The first end 502 of the first member 501 is joined tothe first end 505 of the second member 504 by a first curved member 535to form a first loop 550, the second end 506 of the second member 504 isjoined to the second end 509 of the third member 508 by a second curvedmember 536, and the first end 508 of the third member 507 is joined tothe first end 511 of the fourth member 510 by a third curved member 537to form a second loop 531. The first loop 530 defines a first angle 543.The second loop 531 defines a second angle 544. Each cell 500 alsoincludes a fifth member 513 having a first end 514 and a second end 515;a sixth member 516 having a first end 517 and a second end 518; aseventh member 519 having a first end 520 and a second end 521; aneighth member 522 having a first end 523 and a second end 524; a ninthmember 525 having a first end 526 and a second end 527; and a tenthmember having a first end 529 and a second end 530. The first end 514 ofthe fifth member 513 is joined to the second end 503 of the first member501 at second junction point 542, the second end 515 of the fifth member513 is joined to the second end 518 of the sixth member by a curvedmember 539 to form a third loop 532, the first end 517 of the sixthmember 516 is joined to the first end 520 of the seventh member 519 by afifth curved member 548, the second end 521 of the seventh member 519 isjoined to the second end 524 of the eighth member 522 at third junctionpoint 540 to form a fourth loop 533, the first end 523 of the eighthmember 522 is joined to the first end 526 of the ninth member 525 by asixth curved member 549, the second end 526 of the ninth member 525 isjoined to the second end 530 of the tenth member 528 by a seventh curvedmember 541 to form a fifth loop 534, and the first end 529 of the tenthmember 528 is joined to the second end 512 of the fourth member 510. Thethird loop 532 defines a third angle 545. The fourth loop 533 defines afourth angle 546. The fifth loop 534 defines a fifth angle 547.

[0044] In the embodiment shown in FIG. 4, the first member 501, thethird member 507, the sixth member 516, the eighth member 522, and thetenth member 528 have substantially the same angular orientation to thelongitudinal axis of the stent and the second member 504, the fourthmember 510, the fifth member 513, the seventh member 519, and the ninthmember 512 have substantially the same angular orientation to thelongitudinal axis of the stent. In the embodiment shown in FIG. 4, thelengths of the first, second, third and fourth members 501, 504, 507,510 are substantially equal. The lengths of the fifth, sixth, seventh,eighth, ninth and tenth members 513, 516, 519, 522, 525, 528 are alsosubstantially equal. Other embodiments where lengths of individualmembers are tailored for specific applications, materials ofconstruction or methods of delivery are also possible, and may bepreferable for them.

[0045] Preferably, the first, second, third, and fourth members 501,504, 507, 510 have a width that is greater than the width of the fifth,sixth, seventh, eighth, ninth, and tenth members 513, 516, 519, 522,525, 528 in that cell. The differing widths of the first, second, third,and fourth members and the fifth, sixth, seventh, eighth, ninth, andtenth members with respect to each other contribute to the overallflexibility and resistance to radial compression of the cell. The widthsof the various members can be tailored for specific applications.Preferably, the fifth, sixth, seventh, eighth, ninth, and tenth membersare optimized predominantly to enable longitudinal flexibility, bothbefore and after expansion, while the first, second, third, and fourthmembers are optimized predominantly to enable sufficient resistance toradial compression to hold a vessel open. Although specific members areoptimized to predominantly enable a desired characteristic, all theportions of the cell interactively cooperate and contribute to thecharacteristics of the stent.

[0046]FIGS. 5 and 6 show a pattern and an expanded view of one cell ofan embodiment of the present invention which is specially adapted for astent made of stainless steel. The pattern is similar to the pattern ofFIGS. 3 and 4, and the same reference numerals are used to indicate thegenerally corresponding parts.

[0047] In this embodiment of the invention, for example, the secondloops 531 are made stronger by shortening the third and fourth members507, 510. This helps assure that the second loops do not “flare out”during delivery of the stent through tortuous anatomy. This “flaringout” is not a concern with NiTi stents which are covered by a sheathduring delivery.

[0048] Furthermore, the length of the members in this embodiment may beshorter than the length of the corresponding members in the embodimentillustrated in FIGS. 3 and 4. Typically, the amount of strain allowed ina self-expanding NiTi stent may be around 10%. In a stainless steelstent, the amount of strain allowed typically may be 20% or greater.Therefore, to facilitate stents made of NiTi and stents made ofstainless steel expanding to comparable diameters, the members of theNiTi stent may be longer than the members of a stainless steel stent.

[0049]FIG. 7 illustrates another aspect of the present invention. Thestent of FIG. 7 is also constructed from orthogonal meander patterns301, 302. The meander patterns form a series of interlocking cells 50,700 of two types. The first type of cell 50 is taught by U.S. Pat. No.5,733,303. These cells are arranged so that they form alternating bands704 of first type of cells 50 and bands 706 of the second type of cells700.

[0050] As seen in FIG. 8 and particularly with respect to the celllabeled for ease of description, each of the '303 cells 50 has a firstlongitudinal apex 100 and a second longitudinal end 78. Each cell 50also is provided with a first longitudinal end 77 and a secondlongitudinal apex 104 disposed at the second longitudinal end 78. Eachcell 50 also includes a first member 51 having a longitudinal componenthaving a first end 52 and a second end 53; a second member 54 having alongitudinal component having a first end 55 and a second end 56; athird member 57 having a longitudinal component having a first end 58and a second end 59; and a fourth member 60 having a longitudinalcomponent having a first end 61 and a second end 62. The stent alsoincludes a first loop or curved member 63 defining a first angle 64disposed between the first end 52 of the first member 51 and the firstend 55 of the second member 54. A second loop or curved member 65defining a second angle 66 is disposed between the second end 59 of thethird member 57 and the second end 62 of the fourth member 60 and isdisposed generally opposite to the first loop 63. A first flexiblecompensating member (or a section of a longitudinal meander pattern) 67having curved portion and two legs with a first end 68 and a second end69 is disposed between the first member 51 and the third member 57 withthe first end 68 of the first flexible compensating member 67 joined toand communicating with the second end 53 of the first member 51 and thesecond end 69 of the first flexible compensating member 67 joined to andcommunicating with the first end 58 of the third member 57. The firstend 68 and the second end 69 are disposed a variable longitudinaldistance 70 from each other. A second flexible compensating member (or,a section of a longitudinal meander pattern) 71 having a first end 72and a second end 73 is disposed between the second member 54 and thefourth member 60. The first end 72 of the second flexible compensatingmember 71 is joined to and communicates with the second end 56 of thesecond member 54 and the second end 73 of the second flexiblecompensating member 71 is joined to and communicates with the first end61 of the fourth member 60. The first end 72 and the second end 73 aredisposed a variable longitudinal distance 74 from each other. In thisembodiment, the first and second flexible compensating members, andparticularly the curved portion thereof, 67 and 71 are arcuate.

[0051] The second type of cell 700 is illustrated in FIG. 9 and the samereference numerals are used to indicate generally corresponding areas ofthe cell. The apices 100, 104 of the second type of cell 700 are offsetcircumferentially. Also, each flexible compensating member 67, 71includes: a first portion or leg 79 with a first end 80 and a second end81; a second portion or leg 82 with a first end 83 and a second end 84;and a third portion or leg 85 with the first end 86 and a second end 87,with the second end 81 and the second end 84 being joined by a curvedmember and the first end 83 and the first end 86 being joined by acurved member. The first end of a flexible compensating member 67, 71 isthe same as the first end 80 of the first portion 79, and the second endof a flexible compensating member 67, 71 is the same as the second end87 of the third portion 85. A first area of inflection 88 is disposedbetween the second end 81 of the first portion 79 and the second end 84of the second portion 82 where the curved portion joining them lies. Asecond area of inflection 89 is disposed between the first end 83 of thesecond portion 82 and the first end 86 of the third portion 85 where thecurved portion joining them lies.

[0052] While FIG. 7 illustrates a pattern of alternating bands of cells,the stent may be optimized for a particular usage by tailoring theconfiguration of the bands. For example, the middle band of the secondtype of cells 700 may instead be formed of cells 50, or vice versa. Thesecond type of cells in FIG. 7 may also utilize the cell configurationsdescribed with respect to FIGS. 4 and 6. The cell configurations ofFIGS. 4 and 6 provide the advantage that they will not cause any torqueof one portion of the cell relative to another portion of the cell aboutthe longitudinal axis of the stent upon expansion, which may happen whenthe second type of cells 700 expand, a torque which could cause a stentto deform, and stick out.

[0053] As illustrated in FIG. 7, all of the flexible compensatingmembers are arranged so that the path of the flexible compensatingmembers, from left to right, travels in a generally downward direction.The cells 700 can also be arranged so that the flexible compensatingmembers in one band are arranged in a generally upward direction, andthe flexible compensating members in an adjacent band are arranged in agenerally downward direction. One skilled in the art can easily makethese modifications.

[0054]FIG. 10 is a schematic representation comparing the cells 804 ofthe present invention, which have three points where the intertwinedfirst and second meander patterns meet and are in that sense threecornered or triangular cells, with cells 802 of the '303 stent whichhave four points where the intertwined first and second meander patternsmeet and are in that sense four cornered or square cells. Moreparticularly, on the left side of FIG. 10, a pair of vertical meanderpatterns 806, 826 are joined by members 808, 810, 812 (which aresections of longitudinal meander patterns) to form a plurality of threecornered or triangular cells 804. By triangular cell, it is meant thatthere are three sections 810, 812, 814, each having loop portions andthree associated points 816, 818, 820 of their joining, forming eachcell.

[0055] On the right side of FIG. 10, a pair of vertical meander patterns822, 824 are joined together compensating members 828, 830, 832, 834(which are sections of a longitudinal meander) to form a plurality ofsquare cells 804. By square cell, it is meant that there are foursections, each having loop portions, and four associated points of theirjoining, forming each cell. For example, the shaded cell 802 is formedfrom four sections 832, 836, 830, 838, with four associated points oftheir joining 840, 842, 844, 846.

[0056] Both the square cell and the triangular cell have two kinds ofsections with loops. The first kind of loop containing section is formedfrom a vertical meander pattern and is optimized predominantly to enableradial support. The second kind of loop containing section is optimizedpredominantly to enable flexibility along the longitudinal axis of thestent.

[0057] Although each loop containing section is optimized predominantlyto enable a desired characteristic of the stent, the sections areinterconnected and cooperate to define the characteristics of the stent.Therefore, the first kind of loop containing section contributes to thelongitudinal flexibility of the stent, and the second kind of loopcontaining section contributes to the radial support of the stent.

[0058] In the square cell 802, it can be seen that the second kind ofloop containing sections 830, 832 each have one inflection point 848,850. In the triangular cell, the loop containing sections 810, 812 eachhave two inflection point areas 852, 854, 856, 858. The higher number ofinflection points allows more freedom to deform after expansion of thestent and distributes the deformation over a longer section, thus,reducing the maximal strain along these loop containing sections.

[0059] Furthermore, it can be seen that a square cell 802 is generallymore elongated along the longitudinal axis of the stent than atriangular cell 804, which is generally more elongated along thecircumference of the stent. This also contributes to higher flexibilityafter expansion.

[0060] If the first meander patterns 806, 822, 824, 826 of both types ofcells are constructed identically and spaced apart by the same amount,the area of a triangular cell 804 is the same as a square cell 802. Thiscan be more readily understood with reference to a band of cells aroundthe circumference of a stent. Each band will encompass the same area,and each band will have the same number of cells. Accordingly, the areaof each cell in one band formed of square cells will be the same as thearea of each cell in another band formed of triangular cells.

[0061] Although the areas of the cells are equal, the perimeter of thetriangular cell is larger than the perimeter of the square cell.Therefore, in comparison to a square cell, a triangular cell offersincreased coverage of a vessel wall.

[0062] In the particular embodiments described above, the stent issubstantially uniform over its entire length. However, otherapplications where portions of the stent are adapted to providedifferent characteristics are also possible. For example, as shown inFIG. 11, a band of cells 850 may be designed to provide differentflexibility characteristics or different radial compressioncharacteristics than the remaining bands of cells by altering the widthsand lengths of the members making up that band. Or, the stent may beadapted to provide increased access to a side branch lumen by providingat least one cell 852 which is larger in size then the remaining cells,or by providing an entire band of cells 854 which are larger in sizethan the other bands of cells. Or, the stent may be designed to expandto different diameters along the length of the stent. The stent may alsobe treated after formation of the stent by coating the stent with amedicine, plating the stent with a protective material, plating thestent with a radiopaque material, or covering the stent with a material.

[0063]FIGS. 12 and 13 show alternative patterns for a stent constructedaccording to the principles of the present invention. The stent shown inFIG. 12 has two bands of cells 856 located at each of the proximal end860 and distal and 862. The cells that form the bands of cells 856located at the ends of the stent are '303 type cells. The remainingcells in the stent are the same as described with respect to the cells500 depicted in FIG. 6. The stent shown in FIG. 13 has alternating bandsof cells 864, 866, 868. The first type of band of cells 864 is composedof '303 type cells. The second and third types of bands of cells 866,868 are formed of the cells described with respect to the cells 500depicted in FIG. 4. Of course, any various combination of cells may beused in the present invention.

[0064] Thus, what is described is a longitudinally flexible stent thatutilizes a closed cell structure to provide excellent coverage of thevessel wall. The general concepts described herein can be utilized toform stents with different configurations than the particularembodiments described herein. For example, the general concepts can beused to form bifurcated stents. It will be appreciated by personsskilled in the art that the present invention is not limited to what hasbeen particularly shown and described above. Rather, the scope of thepresent invention is defined by the claims which follow.

What is claimed is:
 1. A multicellular stent comprising: a plurality ofbands of square cells, each square cell including a first loop disposedgenerally longitudinally opposite a second loop, and first pair offlexible compensating members joined to the legs of the first and secondloops; a plurality of bands of triangular cells, each triangular cellcomprising a first loop containing section arranged generally in thecircumferential direction, a second loop containing section connected tothe first loops containing section, and a third loop containing sectionconnected to the first loop containing section and the second loopcontaining section, wherein each band of cells at the ends of the stentare formed of square cells.
 2. A stent according to claim 1 , whereinthe two bands of cells at each end of the stent is formed of squarecells.
 3. A stent according to claim 1 , wherein the stent is formed oftwo adjacent bands of triangular cells alternating with one band ofsquare cells.